Sulfur trioxide delivery system

ABSTRACT

A process for reversible sorption of sulfur trioxide onto a sorbent comprising a) contacting from about 15% to 100% sulfur trioxide with the sorbent under anhydrous conditions at a temperature of from about 35° C. to about 150° C. thereby sorbing the sulfur trioxide onto the sorbent, b) desorbing sulfur trioxide from the sorbent at a temperature of from about 150° C. to about 350° C. at about atmospheric pressure, or under a vacuum pressure, and c) recycling said sorbent by continuously repeating steps a) and b), wherein said sorbent consists essentially of silica or zeolite having a silicon to aluminum ratio in the ranges of from about 1 to about 4.4 or greater than about 5.1, and having a pore size of at least 0.5 nm is disclosed.

FIELD OF THE INVENTION

The present invention relates to a process for the reversible sorptionof sulfur trioxide on a recyclable sorbent, and to the composition ofsorbent and sulfur trioxide.

BACKGROUND OF THE INVENTION

Today the major uses of sulfur trioxide are in sulfonations and themanufacture of explosives. A more recent and relatively smaller volumeuse is in the electronics industry where very pure sulfur trioxide canbe used for etching of electronics parts. Levenson and Waleh describethis application in U.S. Pat. No. 5,763,016, where exposure of theelectronics parts at room temperature to 400° C. to dry gaseous sulfurtrioxide is used to etch organic coatings, films, and layers, includingphotoresists. In this application, the delivery of small quantities ofpure sulfur trioxide is necessary. The electronics industry is typicallynot equipped to handle sulfur trioxide in bulk liquid form, or theconsequences of safety incidents with bulk quantities of such a toxicand corrosive in gas or liquid form.

Liquid sulfur trioxide should be stored at a fairly precise temperaturerange of 35-41° C. to maintain a liquid state and keep it from freezing.Also, to avoid the formation of alpha and beta forms, it is necessary tostore the sulfur trioxide in the presence of a polymerization inhibitor.Temperatures of 35-41° C. result in a significant sulfur trioxide vaporpressure and thus require storage in a pressure vessel. If sulfurtrioxide were sorbed on a substrate, it could be stored under an inertatmosphere at room temperature in a non-pressure container made of asuitable material such as stainless steel or glass. No inhibitor wouldbe required to prevent polymerization.

Tsvetkov, et al., in Khim. Ind. (Sofia) (1987), 59(8), 356-9, Chem.Tech. (Leipzig) (1995), 47(5), 252-4, and Deposited Doc (1983), IssueVINITI, pp. 2873-2880 (1980) describe the sorption of sulfur oxides onacid-modified natural bentonite and on the natural zeolites mordeniteand clinoptilolite (having a SiO₂/Al₂O₃ ratio corresponding to a Si/Alratio equal to 4.5-5.025). They noted the catalytic effect on conversionof sulfur dioxide to sulfur trioxide and that desorption of sulfurtrioxide formed in the sorbent was reversible and not accompanied bychemical transformations. Sorption capacity fell by 20-30% in the first7-8 cycles in the bentonite example. Kel'tsev et al. (Russian J. Phys.Chem. 44(6), 1592-1594, 1970) describe the sorption of sulfur trioxideby hydroxylated and dehydroxylated silica gel.

Typically, the cyclic sorption of sulfur trioxide by acidic zeolitessuch as mordenite deal with zeolites that require relatively hightemperatures of greater than 400° C. for complete desorption. They showa pronounced decrease in activity after several cycles.

It is desirable to have sulfur trioxide in a safer and more easily usedform, such as reversibly sorbed on a substrate that allows easydesorption at lower temperatures and thus simple delivery of the sulfurtrioxide. Also, it is desirable to have the sorbed sulfur trioxide in aneasily flowable form, for instance as free-flowing pellets, available ina commercial quality (typically 98% minimum) for conventional uses or ina highly purified quality for use in the electronics industry (typically99.9%). The desorbed sulfur trioxide should be no lower in purity thanthe sulfur trioxide feedstock. Furthermore, it is desirable for thesorbent to be reusable. The present invention provides such an sorbentand a process for its use.

SUMMARY OF THE INVENTION

The present invention comprises a process for reversible sorption ofsulfur trioxide onto a sorbent comprising a) contacting from about 15%to 100% sulfur trioxide with the sorbent under anhydrous conditions at atemperature of from about 35° C. to about 150° C. thereby sorbing thesulfur trioxide onto the sorbent, b) desorbing sulfur trioxide from thesorbent at a temperature of from about 150° C. to about 350° C. at aboutatmospheric pressure, or under a vacuum pressure, and c) recycling saidsorbent by continuously repeating steps a) and b), wherein said sorbenthas a pore size of at least 0.5 nm and consists essentially of silica orzeolite, said zeolite having a silicon to aluminum ratio in the rangesof from about 1 to about 4.4 or greater than about 5.1.

The present invention further comprises a sorbent consisting essentiallyof silica or zeolite, said zeolite having a silicon to aluminum ratio inthe ranges of from about 1 to about 4.4 or greater than about 5.1, saidsorbent having a pore size of at least 0.5 nm, and having adsorbedthereon a minimum of about 1% by weight sulfur trioxide.

BRIEF DESCRIPTION OF FIGURE

FIG. 1 shows X-ray spectra of an example sorbent before the first sulfurtrioxide sorption (denoted as “A”) and after ten sulfur trioxidesorption and desorption cycles (denoted as “B”). Units of intensity onthe ordinate are counts and the 2-theta abscissa is the scattering anglein degrees.

DETAILED DESCRIPTION

The present invention comprises a process for the reversible sorptionand desorption of sulfur trioxide onto certain molecular sieves orsorbents with recycle of the sorbent. This process is used to providethe composition of the present invention, sorbent having sulfur trioxidesorbed thereon, to an end user requiring a source of sulfur trioxide.Thus, the present invention further comprises a sorbent having a poresize of at least 5 nm, and comprising a silicon or zeolite, said zeolitehaving a silica to aluminum ratio of from about 1 to about 4.4 or about5.1 or greater, said sorbent having adsorbed thereon a minimum of about1% by weight sulfur trioxide. When charged, the sorbent contains fromabout 3% to about 60% by weight sorbed sulfur trioxide, preferably fromabout 10% to about 45% by weight sorbed sulfur trioxide, and mostpreferably from about 15% to about 45% by weight sorbed sulfur trioxide.The sulfur trioxide, sorbed on the sorbent, and in a suitable container,is readily desorbed. It exhibits substantially reduced hazards intransportation and use.

By the term “sorbed” as applied to the process and composition of thisinvention is meant a composition of substrate and sulfur trioxideexhibiting a partial vapor pressure of sulfur trioxide less that that ofsulfur trioxide itself, e.g., at 24° C. a partial vapor pressure of lessthan 0.3 atmosphere (29 kPa).

The composition of the present invention is prepared by step a) of theprocess of the present invention. Sulfur trioxide, of purity from about98% to 100%, is sorbed onto a sorbent. Any source of sulfur trioxide ofadequate purity for the intended end-use may be used, typically acontainer of pure liquid sulfur trioxide is used. The sulfur trioxide,as vapor or liquid, is passed through a bed of the dried sorbent togenerate the sorbent/sulfur trioxide composition of this invention at atemperature in the range of from about 35 to about 150° C., preferablyfrom about 50 to about 125° C., and most preferably in the range of fromabout 70 to about 100° C. The sorbent container optionally may be heatedup to about 150° C. during the sorption or optionally heated and thencooled to increase sorption. The sorbent is contained in any suitablecontainer inert to sulfur trioxide and suitable for the intendeddesorption temperatures. Materials of construction for containing sulfurtrioxide are well known to those skilled in the art. Steel or stainlesssteel cylinders, which may be lined with an inert lining such aspoly(tetrafluoroethylene), are preferred. Optionally an inert carriergas may be used to move the sulfur trioxide into the sorbent. In atypical sorption step, for example, dry nitrogen may be passed throughliquid sulfur trioxide maintained at 35° C. to provide a gas streamcontaining about 50% by volume of sulfur trioxide. The sulfur trioxideconcentration in the feed stream during the sorption step is preferablyat least 15% by volume, the remainder being the inert carrier gas.

By the term “inert carrier gas” is meant a gas that is unreactive withsulfur trioxide, sorbent, or container, and is typically dry nitrogen.When an inert carrier gas is used, the purity of the sulfur trioxide isdescribed exclusive of the carrier gas. Optionally the sulfur trioxidestream can be sorbed under a positive pressure to accelerate sorption.

The present invention uses thermally stable and dry sorbents such assilicalites, zeolites, clays, and silicas to provide long-termcyclability while allowing the sulfur trioxide to be expelled atrelatively low temperatures, up to about 350° C. For example, X-rayspectra of the H-ZSM-5 extrudates as used in Example 21 are shown inFIG. 1 prior to the first sorption of sulfur trioxide (“Unused” trace,denoted as “A”) and again after ten sorption and desorption cycles(“After 10 Cycles” trace, denoted as “B”). The latter trace has beendisplaced upward by 2000 intensity units so that the traces can bedistinguished. The two traces are essentially unchanged, indicating nodeterioration of the sorbent molecular structure. Zeolites with a lowSi/Al ratio (high in Al₂O₃) are prone to structural degradation aftersulfur trioxide sorption and desorption, particularly at highertemperatures. Such structure degradation results in an irreversible lossof sorption capacity for sulfur trioxide, and the structural changesresult in the appearance of a broad peak in the 2-theta range 15-30°.Appearance of a broad peak is clearly absent in the “After 10 Cycles”trace B in FIG. 1.

Molecular sieves, both natural and synthetic, are well known in the artand are defined in R. Szostak, Molecular Sieves—Principles of Synthesisand Identification, Van Nostrand Reinhold, page 2 (1989). The inorganicmolecular sieves used for sorbing and desorbing sulfur trioxide inaccordance with this invention include various silicates (e.g.,titanosilicates, borosilicates, silicalites, low alumina-containingzeolites such as mordenite and ZSM-5, and high alumina-containingzeolites such as 5A, NaY and 13X). The preferred molecular sieves usefulas sorbents in the invention are either acidic or are non-acidicsilicates.

Zeolites can be generically described as complex aluminosilicatescharacterized by a three-dimensional framework structure enclosingcavities occupied by ions and water molecules, all of which can movewith significant freedom within the zeolite matrix. In commerciallyuseful zeolites, the water molecules can be removed from or replacedwithin the framework without destroying its structure. Zeolites can berepresented by the following formula: M_(2/n)O.Al₂O₃.xSiO₂.yH₂O, whereinM is a cation of valence n, x is greater than or equal to 2, and y is anumber determined by the porosity and the hydration state of thezeolite, generally from 0 to 8. In naturally occurring zeolites, M isprincipally represented by Na, Ca, K, Mg and Ba in proportions usuallyreflecting their approximate geochemical abundance. The cations M areloosely bound to the structure and can frequently be completely orpartially replaced with other cations or hydrogen by conventional ionexchange.

The zeolite structure is a corner-linked tetrahedra with Al or Si atomsat centers of tetrahedra and oxygen atoms at corners. Such tetrahedraare combined in a well-defined repeating structure comprising variouscombinations of 4-, 6-, 8-, 10-, and 12-membered rings. The resultingframework is one of regular channels and cages, which impart a usefulpore structure for separation. Pore dimensions are determined by thegeometry of the aluminosilicate tetrahedra forming the zeolite channelsor cages, with nominal openings of 0.26 nm for 6 rings, 0.40 nm for 8rings, 0.55 μm for 10 rings and 0.74 nm for 12 rings (these numbersassume ionic radii for oxygen). Those skilled in the art will recognizethat zeolites with the largest pores being 8 rings, 10 rings, and 12rings are considered small, medium, and large pore zeolites,respectively. Pore dimensions are critical to the performance of thesematerials in catalytic and separation applications, since thischaracteristic determines whether reactant molecules can enter andproduct molecules (in the catalytic application case) can exit thezeolite framework. In practice, it has been observed that very slightdecreases in ring dimensions can effectively hinder or block movement ofparticular reactants or catalysis products within a zeolite structure.

The pore dimensions that control access to the interior of the zeoliteare determined not only by the tetrahedra forming the pore opening, butalso by the presence or absence of ions in or near the pore. In the caseof zeolite A, for example, access can be restricted by monovalent ions,such as Na⁺ or K⁺, which are situated in or near 8-ring openings as wellas 6-ring openings. Access is enhanced by divalent ions, such as Ca²⁺,which are situated only in or near 6-ring openings. Thus, the potassiumand sodium salts of zeolite A exhibit effective pore openings of about0.3 nm and 0.4 nm respectively, whereas the calcium salt of zeolite Ahas an effective pore opening of 0.5 nm. For this application it isimportant that the pore opening be of sufficient size (at least 0.5 nm)to allow the ingress and egress of sulfur trioxide. The presence orabsence of ions in or near the pores, channels, and/or cages can alsosignificantly modify the accessible pore volume of the zeolite forsorbing materials. To maximize capacity, generally protons or smallcations are preferred.

Zeolites are available from various sources. A comprehensive listing ofzeolites vendors is contained in “CEH Marketing Research Report:Zeolites” by M. Smart and T. Esker with A. Leder and K. Sakota, 1999,Chemical Economics Handbook-SRI International.

Low alumina-containing zeolites can be prepared synthetically (e.g.,mordenite, ZSM-5, silicalite) or by modification of highalumina-containing zeolites using methods well known in the art. Thesemethods include but are not limited by treatment using SiCl₄ or(NH₄)₂SiF₆ as well as steaming followed by acid treatment. The SiCl₄treatment is described in J. Chem. Ed. 67(6), 519-521, 1990. The(NH₄)₂SiF₆ treatment by Breck et al., is described in U.S. Pat. No.4,503,023. These treatments are generally very effective at increasingthe Si/Al ratio for zeolites such as zeolites Y and mordenite.

Acid forms of molecular sieve sorbents can be prepared by a variety oftechniques including ammonium exchange followed by calcination or bydirect exchange of alkali ions for protons using mineral acids or ionexchangers (for a discussion of acid sites in zeolites see J. Dwyer,“Zeolite, Structure, Composition and Catalysis” in Chemistry andIndustry, Apr., 2, 1984). Binders for molecular sieve particles may beused as long as they do not affect the molecular sieve's ability to sorband desorb sulfur trioxide.

The silicalites, zeolites (aluminosilicates), titanosilicates,aluminophosphates, silicas, clays, and borosilicates are all suitablefor use as sorbents in the present invention. Examples of suitablesilicalites are silicalite-1 and silicalite-2. Examples of suitablezeolites (aluminosilicates) are mordenite, Y, X, 5A, US-Y, DA-Y, ZSM-5,ZSM-11, beta, L, ferrierite, and clinoptilolite. Examples of suitabletitanosilicates are TS-1, TS-2, and Ti-beta. Examples of suitable claysare bentonite, montmorillonite, kaolin, and talc. Examples of suitableborosilicates are boralite-A, boralite-B, boralite-C, and boralite-D.Examples of suitable aluminophosphates are AlPO₄-5, SAPO-5, AlPO₄-11,SAPO-34, and so on. Silicas include precipitated silica, dried silicasols, diatomaceous earth, silica gels, and fumed silicas. Preferredsorbent materials include high surface area silicas and the highsilica-containing molecular sieve materials (Si/Al greater than about5.1) prepared either by synthesis or modification. These materialsinclude silicalite, mordenite, beta, US-Y, DA-Y, ZSM-5, ZSM-11,borosilicates, titanosilicates and the like. The most preferred sorbentmaterials have a Si/Al ratio of at least about 25. Sorbent materialswith Si/Al ratios in the range from about 1 to about 4.4 can also beused, but are less preferred as the sulfur trioxide/sorbent structurebecomes less structurally stable to higher temperatures as this ratiodecreases. The amount of sulfur trioxide sorbed is at least about 1%,preferably at least about 3%, and most preferably at least about 5% byweight, based on the weight of the sorbent. The maximum amount sorbed isdependent upon the physical structure of the sorbent used, typically inthe range from about 40% to about 60% based on the weight of thesorbent.

Additionally and preferably, the sorbent is used in a pelletized,beaded, or extruded and chopped form to facilitate gas or liquid flowthrough the sorbent bed. The typically powdered sorbent may bepelletized using suitable binder. Binders must be stable to exposure tosulfur trioxide and the sorption/desorption conditions. Gamma-alumina,silica, and clays are examples of suitable binders. The pelletization ofsuch sorbents is well known to those skilled in the art and providespellets that remain free flowing after repeated sorption and desorptioncycles.

The use of the present invention is not limited to the supply of puresulfur trioxide for such specialized applications as etching for theelectronics industry. Larger volume conventional users of sulfurtrioxide, such as in sulfonations, must manage the hazards presented bybulk storage of sulfur trioxide. The sorbed sulfur trioxide compositionof the present invention provides a major hazards management method forall users.

Typical specifications for the sulfur trioxide feed during the sorptionstep has a sulfur trioxide content equal or greater than 99.5% andpreferably equal or greater than 99.9%. The sulfur trioxide feedcontains equal or less than 0.4% by weight sulfuric acid and preferablyequal or less than 0.1%, and has an iron (measured as Fe) concentrationof less than 5 parts per million by weight (ppm) and preferably equal orless than 0.7 ppm. Desorbed sulfur trioxide quality corresponds to thequality of the sorbed sulfur trioxide.

In the process of this invention, the sorbent/sulfur trioxidecomposition can be stored indefinitely and transported under ambienttemperature conditions in a dry container of inert materials ofconstruction as described above.

At a site requiring the safe delivery of small amounts of sulfurtrioxide, a container holding the sorbent/sulfur trioxide composition issubjected to 1) controlled heating (at from about 150 to about 350° C.,termed heat-assisted desorption) and/or 2) vacuum pressures down toabout 100 mm (13 kPa), (termed vacuum-assisted desorption) to provide asource of pure sulfur trioxide. Temperatures are given for heat-assisteddesorption at, or close to, atmospheric pressure. Temperatures forvacuum-assisted desorption are correspondingly lower. When the sorbedsulfur trioxide is depleted, as indicated by cessation of the flow ofsulfur trioxide, the depleted sorbent is then be recharged with puresulfur trioxide and thus recycled. Desorption may be facilitated bydesorption at reduced pressure, use of an inert carrier gas, or both.Various methods for controlled heating may be used, such as ovens,heating jackets, or the heat may be supplied by using a heated inert gascarrier and insulating the container.

In a typical application the initial charging of sulfur trioxide ontothe sorbent and subsequent recharging is preferably performed by aprovider at a site equipped for the safe handling of bulk quantities ofsulfur trioxide. The user, on the other hand, receives suitablecontainers, e.g., cylinders, of the charged sulfur trioxide-sorbent intothe appropriate reaction area, where the container is heated and can besubjected to vacuum to desorb pure sulfur trioxide as required. The userthen recycles the sorbent to the provider. The user is thereby relievedof the problems of safe handling of bulk sulfur trioxide.

EXAMPLES

Examples 1-16 and Comparative Example A demonstrate the sorption ofsulfur trioxide with various sorbents. Thermogravimetric analysis wasused to characterize desorption from low temperature sites (wheredesorption occurs at less than 350° C.) and total of low and hightemperature sites (where desorption occurs at less than 750° C.). Suchcharacterization distinguishes the more useful and practical sites(where heat-assisted desorption occurs at less than 350° C.) from thosethat have a significant part of the sulfur trioxide sorbed on hightemperature sites (where heat-assisted desorption occurs at 350-750°C.). In most applications, only the low temperature sites (less than350° C.) will be used to minimize hazards and facilitate handling.Sulfur trioxide bound at the higher temperature sites remains on thesupport. It should be noted that the thermogravimetric analysisprocedure used in these small scale evaluations does not provideparticularly accurate or consistent weight changes during desorption dueto such factors as variable moisture sorption as the charged sorbentsample is prepared.

Examples 17 and 18 demonstrate the process of repeated sorption anddesorption cycles. Examples 17 and 18 are on a larger scale anddesorption does not involve exposure of the charged sorbent to ambientconditions. Thus weight changes in Examples 17 and 18 are indicative ofthe sorption and desorption capacity for sulfur trioxide.

Example 1

A sample of 5A powder (Molecular Sieve Type 5A, from Linde Division ofUnion Carbide, New York N.Y.) was calcined in air by raising thetemperature 60° C. per hour to 500° C. and holding at 500° C. for 5hours. The sample was cooled to 110° C. and transferred to a dried vial.A poly(tetrafluoroethylene) vessel was loaded with 0.53 g of the dried5A powder and heated to approximately 60° C. Distilled sulfur trioxidevapor (44-45° C.) was purged over the solid for 10 minutes. The solidwas then heated to 91° C. under a dry nitrogen purge for 1.25 h toremove surface bound sulfur trioxide. The sample was rapidly cooled toroom temperature. The next day nitrogen flow was restarted and thesystem heated to 94° C. for 45 minutes at which time there was noapparent fuming observed in the scrubber unit. A sample of the 5A/sulfurtrioxide complex was then transferred under anhydrous conditions to athermogravimetric analyzer where it lost 24.21% of its weight betweenroom temperature and 350° C. and 54.81% between room temperature and750° C. under flowing nitrogen.

Example 2

A sample of 13× powder (13×, Lot number 01820CY, Aldrich Chemical,Milwaukee Wis.) was calcined in air by raising the temperature 60° C.per hour to 500° C. and holding at 500° C. for 5 hours. The sample wascooled to 110° C. and transferred to a dried vial. Apoly(tetrafluoroethylene) vessel was loaded with 0.58 g of the dried 13×powder and heated to approximately 60° C. Distilled sulfur trioxidevapor (about 45° C.) was purged over the solid for 10 minutes. The solidwas then heated to about 90° C. under a dry nitrogen purge for 3.5 h toremove surface bound sulfur trioxide. A sample of the 13×/sulfurtrioxide complex was then transferred under anhydrous conditions to athermogravimetric analyzer where it on average lost 29.56% of its weightbetween room temperature and 350° C. and 56.32% between room temperatureand 750° C. under flowing nitrogen.

Example 3

A sample of 13× powder (13×, Lot number 01820CY, see Example 2) wascalcined in air by raising the temperature 60° C. per hour to 500° C.and holding at 500° C. for 5 hours. The sample was cooled to 110° C. andtransferred to a dried vial. A poly(tetrafluoroethylene) vessel wasloaded with 0.51 g of the dried 13× powder and heated to approximately60° C. Distilled sulfur trioxide vapor (about 45° C.) was purged overthe solid for 10 minutes. The solid was then heated to about 90° C.under a dry nitrogen purge for 3.67 h to remove surface bound sulfurtrioxide. A sample of the 13×/sulfur trioxide complex was thentransferred under anhydrous conditions to a thermogravimetric analyzerwhere it on average lost 27.60% of its weight between room temperatureand 350° C. and 55.14% between room temperature and 750° C. underflowing nitrogen.

Example 4

A 20 g sample of silicalite powder (S-115, from Union Carbide, New York,N.Y.) was placed in a quartz tube in a vertically mounted tube furnace,heated by raising the temperature 60° C. per hour to 500° C. and holdingat 500° C. for 5 hours under flowing nitrogen. The sample was cooledunder flowing nitrogen and then transferred to a dry box. Apoly(tetrafluoroethylene) vessel was loaded with 1 g of the driedsilicalite powder and heated to 60° C. Distilled sulfur trioxide vapor(44° C.) was purged over the solid for 1.25 hour. The solid was thenheated to 78° C. under a dry nitrogen purge for 11.5 h to remove surfacebound sulfur trioxide. The final weight of the silicalite/sulfurtrioxide complex was 1.34 g (34.00% weight gain, 25.37% sulfur trioxideloading). The silcalite/sulfur trioxide complex was then transferredunder anhydrous conditions to the thermogravimetric analyzer where it onaverage lost 30.16% of its weight between room temperature and 350° C.and 30.00% between room temperature and 750° C.

Example 5

A 20 g sample of silicalite beads (S-115, see Example 4) was placed in aquartz tube in a vertically mounted tube furnace, heated by raising thetemperature 60° C. per hour to 500° C. and holding at 500° C. for 5hours under flowing nitrogen. The sample was cooled under flowingnitrogen and then transferred to a dry box. A poly(tetrafluoroethylene)vessel was loaded with 3.07 g of the dried silicalite beads. Ten dropsof distilled sulfur trioxide liquid was added to the solid at 35° C. Thesolid was heated to approximately 40° C. under a dry nitrogen purge forapproximately 2 h and then heated to 60° C. for 4.25 h to remove surfacebound sulfur trioxide. The sample was cooled to room temperatureovernight. Heating at 60° C. was resumed the following day forapproximately 7.5 h until fuming in the scrubber was minimized. Thefinal weight of the silicalite/sulfur trioxide complex was 3.61 g(17.59% weight gain, 14.96% sulfur trioxide loading). The complex wasthen transferred under anhydrous conditions to the thermogravimetricanalyzer where the average weight loss was 11.39% of its weight betweenroom temperature and 350° C. and 13.76% between room temperature and750° C.

Example 6

A 20 g sample of NH4-mordenite (Si/Al=15, Valfor CBV-30A, Lot Number30A-HM-6, PQ Corp., Valley Forge Pa.) was placed in a quartz tube in avertically mounted tube furnace, heated by raising the temperature 60°C. per hour to 500° C. and holding at 500° C. for 5 hours under flowingnitrogen. A poly(tetrafluoroethylene) vessel containing 1.00 g ofresulting dried H-Mordenite (Si/Al=15) powder was heated to about 60° C.and distilled sulfur trioxide vapor (about 44° C.) was purged over thesolid for 1.25 hours. The solid was then heated to 78° C. under a drynitrogen purge for 7.65 h to remove surface bound sulfur trioxide. Thefinal weight of the H-Mordenite (Si/Al=15)/sulfur trioxide complex was1.12 g (12% weight gain, 9.71% sulfur trioxide loading). The H-Mordenite(Si/Al=15)/sulfur trioxide complex was then transferred under anhydrousconditions to the thermogravimetric analyzer where the average weightloss was 17.91% of its weight between room temperature and 350° C. and27.50% between room temperature and 750° C.

Example 7

A 20 g sample of DAY (Si/Al=55) (DAY-55, Lot Number TC133, DegussaCorp., Frankfurt, Germany and South Plainfield N.J.) was placed in aquartz tube in a vertically mounted tube furnace, heated by raising thetemperature 60° C. per hour to 500° C. and holding at 500° C. for 5hours under flowing nitrogen. A poly(tetrafluoroethylene) vesselcontaining 1.02 g of dried DAY (Si/Al=55) powder was heated to about 66°C. and distilled sulfur trioxide vapor (about 44° C.) was purged overthe solid for 2 hours. The solid was then heated to 81° C. under a drynitrogen purge for 8.3 h to remove surface bound sulfur trioxide. Thefinal weight of the DAY (Si/Al=55)/sulfur trioxide complex was 1.20 g(17.65% weight gain, 13.71% sulfur trioxide loading). The DAY(Si/Al=55)/sulfur trioxide complex was then transferred under anhydrousconditions to the thermogravimetric analyzer where the average weightloss was 23.48% of its weight between room temperature and 350° C. and27.63% between room temperature and 750° C.

Example 8

A 20 g sample of NH4-mordenite (Si/Al=45) (CBV-90A, Lot Number 1822-42,Zeolyst Corp., Valley Forge Pa.) was placed in a quartz tube in avertically mounted tube furnace, heated by raising the temperature 60°C. per hour to 500° C. and holding at 500° C. for 5 hours under flowingnitrogen. A poly(tetrafluoroethylene) vessel containing 1.03 g of theresulting dried H-mordenite (Si/Al=45) powder was heated to about 64° C.and distilled sulfur trioxide vapor (about 44° C.) was purged over thesolid for 1.25 hours. The solid was then heated to 73° C. under a drynitrogen purge for 3.8 h to remove surface bound sulfur trioxide. Thefinal weight of the H-mordenite (Si/Al=45)/sulfur trioxide complex was1.69 g (64.08% weight gain, 36.92% sulfur trioxide loading). TheH-mordenite (Si/Al=45)/sulfur trioxide complex was then transferredunder anhydrous conditions to the thermogravimetric analyzer where theaverage weight loss was 33.31% of its weight between room temperatureand 350° C. and 39.67% between room temperature and 750° C.

Example 9

A 20 g sample of silica gel (powder, 952-08-5×1950, Davison Division ofW. R. Grace, Baltimore Md.) was placed in a quartz tube in a verticallymounted tube furnace, heated by raising the temperature 60° C. per hourto 500° C. and holding at 500° C. for 5 hours under flowing nitrogen. Apoly(tetrafluoroethylene) vessel containing 1.01 g dried silica gel washeated to about 60° C. and distilled sulfur trioxide vapor (about 44°C.) was purged over the solid for 1.25 hours. The solid was then heatedto 78° C. under a dry nitrogen purge for 4.85 h to remove surface boundsulfur trioxide. The final weight of the silica gel/sulfur trioxidecomplex was 1.69 g (67.33% weight gain, 38.84% sulfur trioxide loading).The silica gel/sulfur trioxide complex was then transferred underanhydrous conditions to the thermogravimetric analyzer where the averageweight loss was 44.79% of its weight between room temperature and 350°C. and 45.92% between room temperature and 750° C.

Example 10

A 20 g sample of silica gel (powder, 952-08-5×1950, Davison Division ofW. R. Grace, Baltimore Md.) was placed in a quartz tube in a verticallymounted tube furnace, heated by raising the temperature 60° C. per hourto 500° C. and holding at 500° C. for 5 hours under flowing nitrogen. Apoly(tetrafluoroethylene) vessel containing 1.00 g dried silica gel washeated to about 63° C. and distilled sulfur trioxide vapor (about 45°C.) was purged over the solid for 1.25 hours. The solid was then heatedto 73° C. under a dry nitrogen purge for 4.6 h to remove surface boundsulfur trioxide. The final weight of the silica gel/sulfur trioxidecomplex was 1.36 g (36% weight gain, 25.47% sulfur trioxide loading).The silica gel/sulfur trioxide complex was then transferred underanhydrous conditions to the thermogravimetric analyzer where the averageweight loss was 30.52% of its weight between room temperature and 350°.

Example 11

A 20 g sample of silica gel (powder, 952-08-5×1950, Davison Division ofW. R. Grace, Baltimore Md.) was placed in a quartz tube in a verticallymounted tube furnace, heated by raising the temperature 60° C. per hourto 500° C. and holding at 500° C. for 5 hours under flowing nitrogen. Apoly(tetrafluoroethylene) vessel containing 1.00 g dried silica gel washeated to about 62° C. and distilled sulfur trioxide vapor (about 47°C.) was purged over the solid for 1.25 hours. The solid was then heatedto 79° C. under a dry nitrogen purge for 3.2 h to remove surface boundsulfur trioxide. The final weight of the silica gel/sulfur trioxidecomplex was 1.56 g (56% weight gain, 35.9% sulfur trioxide loading). Thesilica gel/sulfur trioxide complex was then transferred under anhydrousconditions to the thermogravimetric analyzer where the average weightloss was 21.25% of its weight between room temperature and 350°.

Example 12

A 20 g sample of silica gel (powder, 952-08-5×1950, Davison Division ofW. R. Grace, Baltimore Md.) was placed in a quartz tube in a verticallymounted tube furnace, heated by raising the temperature 60° C. per hourto 500° C. and holding at 500° C. for 5 hours under flowing nitrogen. Apoly(tetrafluoroethylene) vessel containing 3.01 g dried silica gel washeated to about 57° C. and distilled sulfur trioxide vapor (about 36°C.) was purged over the solid for 1 hour. The solid was then heated to64° C. under a dry nitrogen purge for 2.4 h to remove surface boundsulfur trioxide. The final weight of the silica gel/sulfur trioxidecomplex was 3.51 g (16.61% weight gain, 14.25% sulfur trioxide loading).The silica gel/sulfur trioxide complex was then transferred underanhydrous conditions to the thermogravimetric analyzer where the averageweight loss was 13.55% of its weight between room temperature and 350°.

Example 13

M-5 (Lot Number 962483061001-S-10, UOP Corp., Des Plaines Ill.) wasplaced in a quartz tube in a vertically mounted tube furnace, heated byraising the temperature 60° C. per hour to 500° C. and holding at 500°C. for 5 hours under flowing nitrogen. A poly(tetrafluoroethylene)vessel containing 1.00 g of resulting dried Na-Mordenite (Si/Al=5.35)powder was heated to about 60° C. and distilled sulfur trioxide vapor(about 47° C.) was purged over the solid for 0.17 hours. The solid wasthen heated to 90° C. under a dry nitrogen purge for 4.9 h to removesurface bound sulfur trioxide. The final weight of the Na-Mordenite(Si/Al=5.35)/sulfur trioxide complex was 1.22 g (22% weight gain, 17.03%sulfur trioxide loading). The Na-Mordenite (Si/Al=5.35)/sulfur trioxidecomplex was then transferred under anhydrous conditions to thethermogravimetric analyzer where the average weight loss was 13.10% ofits weight between room temperature and 350° C. and 22.16% between roomtemperature and 750° C.

Example 14

A 15 g sample of Na-mordenite (Si/Al=5.35) (LZ-M-5, Lot Number962483061001-S-10, UOP Corp., Des Plaines Ill.) was placed in a quartztube in a vertically mounted tube furnace, heated by raising thetemperature 60° C. per hour to 500° C. and holding at 500° C. for 5hours under flowing nitrogen. A poly(tetrafluoroethylene) vesselcontaining 1.00 g of resulting dried Na-Mordenite (Si/Al=5.35) powderwas heated to about 60° C. and distilled sulfur trioxide vapor (about45° C.) was purged over the solid for 1 hour. The solid was then heatedto 38° C. under a dry nitrogen purge for 6.6 h to remove surface boundsulfur trioxide. The final weight of the Na-Mordenite(Si/Al=5.35)/sulfur trioxide complex was 1.37 g (37% weight gain, 26.01%sulfur trioxide loading). The Na-Mordenite (Si/Al=5.35)/sulfur trioxidecomplex was then transferred under anhydrous conditions to thethermogravimetric analyzer where the average weight loss was 30.67% ofits weight between room temperature and 350° C. and 40.20% between roomtemperature and 750° C.

Example 15

A 20 g sample of US-Y (Si/Al=2.8) (LZ-20, Lot Number 15228-65, UOPCorp., Des Plaines Ill.) was placed in a quartz tube in a verticallymounted tube furnace, heated by raising the temperature 60° C. per hourto 500° C. and holding at 500° C. for 5 hours under flowing nitrogen. Apoly(tetrafluoroethylene) vessel containing 1.00 g of resulting driedUS-Y (Si/Al=2.8) powder was heated to about 60° C. and distilled sulfurtrioxide vapor (about 45° C.) was purged over the solid for 1.25 hours.The solid was then heated to 86° C. under a dry nitrogen purge for 12.45h to remove surface bound sulfur trioxide. The final weight of US-Y(Si/Al=2.8)/sulfur trioxide complex was 1.26 g (26% weight gain, 19.63%sulfur trioxide loading). The US-Y (Si/Al=2.8)/sulfur trioxide complexwas then transferred under anhydrous conditions to the thermogravimetricanalyzer where the average weight loss was 14.05% of its weight betweenroom temperature and 350° C. and 37.70% between room temperature and750° C.

Example 16

A 9.12 g sample of FCC catalyst (Super Nova D, W. R. Grace Co.,Baltimore Md.) was placed in a quartz tube in a vertically mounted tubefurnace, heated by raising the temperature 60° C. per hour to 550° C.and holding at 500° C. for 5 hours under flowing nitrogen. Apoly(tetrafluoroethylene) vessel containing 2.03 g of resulting driedFCC powder was heated to about 35° C. and distilled sulfur trioxidevapor (about 45° C.) was purged over the solid for 1 hour. The solid wasthen heated to about 83° C. under a dry nitrogen purge for 3.5 h toremove surface bound sulfur trioxide. The final weight of FCC/sulfurtrioxide complex was 2.54 g (25.12% weight gain, 20.08% sulfur trioxideloading). The FCC/sulfur trioxide complex was then transferred underanhydrous conditions to the thermogravimetric analyzer where the averageweight loss was 1.33% of its weight between room temperature and 350Cand 9.97% between room temperature and 750° C.

Example 17

A sample (100 g) of silica gel powder (Lot number 952-08-5×1950, DavisonDivision, W. R. Grace, Baltimore Md.) was placed in a quartz tube in avertically mounted tube furnace, heated by raising the temperature 60°C. per hour to 500° C. and holding at 500° C. for 5 hours under flowingnitrogen. The sample was cooled under flowing nitrogen and thentransferred to a dry box.

(i). Sorption Cycle

A poly(tetrafluoroethylene) vessel was loaded with 20.0 g of the driedsilica gel and heated to approximately 38° C. Distilled sulfur trioxidevapor (about 38° C.) was purged over the solid for 2.67 h. The samplewas also shaken for 30 seconds every 10 minutes. The solid was thenmaintained at 60° C. under a dry nitrogen purge for 4.5 h to removesurface bound sulfur trioxide. The final weight of the silica gel/sulfurtrioxide complex was 23.85 g (19.25% weight gain, 16.14% sulfur trioxideloading). A sample of the silica gel/sulfur trioxide complex was thentransferred under anhydrous conditions to a thermogravimetric analyzerwhere it lost 18.69% of its weight between room temperature and 350° C.and 20.21% between room temperature and 750° C. under flowing nitrogen.

(ii). Desorption Cycle

A sample (9.96 g) of the silica gel/sulfur trioxide complex prepared in(i) above was loaded in a drybox into a quartz tube and then placed in avertically mounted tube furnace. Under flowing nitrogen the material washeated to 350° C. and held for 24 h and then cooled. Effluent gases werepassed through aqueous KOH scrubbers to remove sulfur trioxide. Thefinal weight was 8.24 g (17.27% weight loss). A sample of the desorbedsilica gel/sulfur trioxide complex was then transferred under anhydrousconditions to a thermogravimetric analyzer where it lost 4.87% of itsweight between room temperature and 350° C. and 7.02% between roomtemperature and 750° C. under flowing nitrogen. The loss to 350° C. isan indication of the effectiveness of the desorption procedure. The lossfrom 350 to 750° C. is an indication of amounts of “unusable” sulfurtrioxide.

The sorption and desorption procedures (i) and (ii) above were repeateda total of 4 cycles. The results are summarized in the Table 1.

The sample from the sorption stage for cycle 4S was affected by highhumidity while being transferred to the thermogravimetric analyzerequipment. Thermogravimetric analyses after sorption (1S, 2S, and 3S)typically show negligible weight loss between ambient room temperatureand 150° C. Examination of the thermogravimetric analysis results forSample 4S showed a major weight loss between room temperature and 150°C., attributed to the exposure to humidity. Subtracting this weight lossfrom the weight loss to 350° C. gives the corrected underscored valuesin Table 1.

TABLE 1 Results of Sorption and Desorption Cycles Weight Weight TGA TGATGA Starting Final Gain Gain Loss to Loss to Loss Cycle/ Weight Weight(Loss)^(b) (Loss)^(c) 350° C. 750° C. 350°- Step^(a) (g) (g) (%) (%) (%)(%) 750° C. 1S 20.00 23.85 19.25 16.14 18.69 20.21 1.52 1D 9.96 8.24(17.27) (20.87) 4.87 7.02 2.15 2S 7.79 9.64 23.75 19.19 21.96 24.24 2.282D 8.98 7.08 (21.16) (26.84) 5.58 7.44 1.86 3S 6.88 8.98 30.52 23.3924.33 26.60 2.27 3D 8.41 6.38 (24.14) (31.82) 6.40 9.20 2.80 4S 6.138.05 31.32 23.85 41.3 44.0 2.70 Corrected ^(d) 25.6 ^(d) 28.9 ^(d) 4D7.65 5.45 (28.76) (40.37) 5.880 9.30 3.50 ^(a)S: sorption step; D:desorption step. ^(b)Based on starting weight; samples forthermogravimetric analysis (TGA) were removed at each cycle, thus thestarting weight necessarily decreases. ^(c)Based on final weight.^(d)Underscored values are corrected for humidity exposure duringhandling, as described at the beginning of this example.

The results in Table 1 illustrate the capacity for repeated sorption anddesorption cycles and demonstrate the stability of the silica to suchcycles.

Example 18

A sample (80 g) of an extruded H-ZSM-5 (PQ Corp., Valley Forge Pa.),Si/Al=25, 20% alumina binder) was placed in a quartz tube in avertically mounted tube furnace, heated by raising the temperature 60°C. per hour to 500° C. and holding at 500° C. for 5 hours under flowingnitrogen. The sample was cooled under flowing nitrogen and thentransferred to a dry box.

Cycle 1. Adsorption

A poly(tetrafluoroethylene) vessel was loaded with 30.0 g of the driedH-ZSM-5 extrudates and heated to approximately 60° C. Distilled sulfurtrioxide vapor (about 38° C.) was purged over the solid for 2 h. Thesample was also shaken for 30 seconds every 10 minutes. The solid wasthen maintained at 60° C. under a dry nitrogen purge for 9.25 h toremove surface bound sulfur trioxide. The final weight of the H-ZSM-5extrudate/sulfur trioxide complex was 37.25 g (24.17% weight gain,19.46% sulfur trioxide loading). A sample of the H-ZSM-5extrudate/sulfur trioxide complex was then transferred under anhydrousconditions to a thermogravimetric analyzer where it on average lost14.01% of its weight between room temperature and 350° C. and 18.85%between room temperature and 750° C. under flowing nitrogen.

Cycle 1. Desorption

The H-ZSM-5 extrudate/sulfur trioxide complex (30.03 g) was loaded in adrybox into a quartz tube and then placed in a vertically mounted tubefurnace. Under flowing nitrogen the material was heated to 350° C. andheld for 24 h and then cooled. (Effluent gases were passed throughaqueous potassium hydroxide scrubbers to remove sulfur trioxide.) Thefinal weight was 25.48 g (15.09% weight loss). A sample of the desorbedH-ZSM-5 extrudate/sulfur trioxide complex was then transferred underanhydrous conditions to a thermogravimetric analyzer where it on averagelost 1.14% of its weight between room temperature and 350° C. and 5.07%between room temperature and 750° C. under flowing nitrogen. (The lossto 350° C. is an indication of the effectiveness of the desorptionprocedure. The loss from 350 to 750° C. is an indication of amounts of“unusable” sulfur trioxide.)

The adsorption and desorption procedures were repeated a total of 10cycles. The results are summarized in Table 2 below.

The apparent temporary low capacity in recyle runs 4-6 in Example 21,are due to temperature excursions during the recycle runs. Sulfurtrioxide sorption capacity can vary with sorption temperatures asdiscussed above. Due to a faulty thermocouple and unrelated to theinvention, temperatures were less than required during the threespecified recycle runs in Example 21, temporarily mimicking a capacitydecrease. The thermocouple was replaced in recycle runs 7-10, and sulfurtrioxide capacity measurements returned to the initial value.

TABLE 2 Sorption/Desorption Cycles for H-ZSM-5 Extrudates. Avg. Avg.Avg. % % TGA TGA TGA Loss Wt. Wt. Loss Loss 350° C. Start. Final GainGain to to to Step Wt. Wt. or Loss or Loss 350° C. 750° C. 750° C. Cycle(1) (g) (g) (2) (3) (4,5) (4,5) (4,5) 1 A 30.00 37.25 24.17 19.46 13.2918.2 4.91 1 D 30.03 25.48 (15.09) (17.78) 1.07 5.05 3.98 2 A 22.15 25.2914.20 12.43 11.10 17.29 6.19 2 D 23.00 20.14 (12.43) (14.20) 1.24 5.54.26 3 A 18.68 21.35 14.29 12.51 10.83 17.5 6.67 3 D 19.46 16.95 (12.90)(14.81) 0.85 4.6 3.75 4 (6) A 16.34 18.53 13.40 11.82 11.06 16.39 5.33 4(6) D 16.92 15.69 (7.27) (7.84) 4.42 9.71 5.29 5 (6) A 14.81 15.69 5.945.61 7.30 12.84 5.54 5 (6) D 14.50 13.30 (8.28) (9.02) 2.50 6.78 4.28 6(6) A 12.77 13.43 5.17 4.91 5.30 10.37 5.07 6 (6) D 12.02 11.47 (4.58)(4.80) 0.83 4.91 4.08 7 A 10.91 13.35 22.36 18.28 18.60 24 5.40 7 D11.89 10.00 (15.90) (18.90) 1.50 5.72 4.22 8 A 9.43 11.44 21.31 17.5713.31 18.75 5.44 8 D 10.17 8.60 (15.44) (18.26) 1.24 6.58 5.34 9 A 8.119.63 18.74 15.78 11.25 17.64 6.39 9 D 8.35 7.28 (12.81) 14.70 0.83 6.93610 10  A 6.75 8.75 29.63 22.86 18.06 24.17 6.11 10  D 7.50 5.99 (20.13)(25.21) 0.56 5.14 4.58 (1) “A” denotes the sulfur trioxide sorptionstep, “D” the sulfur trioxide desorption step. (2) Weight gain or lossas % of Starting Weight (losses shown in parentheses). (3) Weight gainor loss as % of Final Weight (losses shown in parentheses). (4) Percentthermogravimetric analysis (TGA) weight loss was measured and is shownin two temperature ranges, ambient to 350° C. (operating range for theinvention) and 350-750° C, (for complete removal of all sulfurtrioxide). (5) Cycle averages were: Average sulfur trioxide loading:14.12 ± 5.82% Average sulfur trioxide delivered: 12.48 ± 4.63% (6) Asindicated above, the apparent temporary low capacity in recyle runs 4-6in Example 21 are due to temperature excursions.

The results in Table 2 illustrate the capacity for repeated sorption anddesorption cycles and demonstrate the stability of the H-ZSM-5 extrudateto such cycles.

Comparative Example A

A 20 g sample of alumina (Al-3945, {fraction (1/12)}″ extrudates, Lot 4,0537-178, Harshaw Corp., Beechwood Ohio) was placed in a quartz tube ina vertically mounted tube furnace, heated by raising the temperature 60°C. per hour to 500° C. and holding at 500° C. for 5 hours under flowingnitrogen. A poly(tetrafluoroethylene) vessel containing 1.00 g driedalumina was heated to about 59° C. and distilled sulfur trioxide vapor(about 44° C.) was purged over the solid for 1.25 hours. The solid wasthen heated to 74° C. under a dry nitrogen purge for 3.8 h to removesurface bound sulfur trioxide. The final weight of the alumina/sulfurtrioxide complex was 1.67 g (67% weight gain, 39.12% sulfur trioxideloading). The alumina/sulfur trioxide complex was then transferred underanhydrous conditions to the thermogravimetric analysis where the averageweight loss was 4.03% of its weight between room temperature and 350° C.and 21.98% between room temperature and 750° C. Comparative Example Ademonstrated the unsuitability of alumina in the practice of thisinvention.

What is claimed is:
 1. A process for reversible sorption of sulfurtroxide onto a sorbent comprising a) at a site equipped for handlingbulk quantities of sulfur trioxide contacting from about 15% to 100%sulfur trioxide in an inert gas with the sorbent under anhydrousconditions at a temperature of from about 35° C. to about 150° C.thereby sorbing the sulfur trioxide onto the sorbent which sorbent isthen capable of storage, and transporting said sorbent in a container toa site requiring delivery of sulfur trioxide, b) at said site requiringdelivery desorbing sulfur trioxide from the sorbent at a temperature offrom about 150° C. to about 350° C. at about atmospheric pressure, orunder a vacuum pressure, and transporting said sorbent to said site forhandling bulk quantities of sulfur trioxide, and c) recycling saidsorben by continuously repeating steps a) and b), wherein said sorbenthas structural stability upon recycle, a pore size of at least 0.5 nm,and consists essentially of silica or zeolite, said zeolite having asilicon to aluminum ratio equal to about 15, or greater.
 2. The processof claim 1 wherein the sulfur trioxide sorbed and desorbed is of purityof from about 99% to 100%.
 3. The process of claim 2, wherein the sulfurtrioxide is of a purity of at least 99.9%.
 4. The process of claim 1wherein the sorbent has sorbed thereon from about 3% to about 60% byweight sulfur trioxide.
 5. The process of claim 1 wherein the sulfurtrioxide is sorbed onto the sorbent at a temperature of from about 50°C. to about 125° C.
 6. The process of claim 1 wherein the sorbent is asilica or a zeolite, wherein said zeolite has a silicon to aluminumratio of at least
 25. 7. A sorbent consisting essentially of silica orzeolite, said zeolite having a silicon to aluminum ratio equal to about15, or greater, said sorbent having a pore size of at least 0.5 nm, andhaving adsorbed thereon a minimum of about 1% by weight sulfur trioxide.8. The sorbent of claim 7 which is a zeolite having a silicon toaluminum ratio of at least
 25. 9. The sorbent of claim 7 in apelletized, beaded or chopped form.